Since you are just installing Vista on a Virtual Machine, I assume you are using VirtualBox.
You may use a retail .iso with Vista Service Pack 1, follow the instructions given for creation of the .iso.
Next install Windows Vista without a product key, you should get to use Windows Vista for 1 month (30 days) without activating. When the time limit runs out you may delete your Virtual Machine and make a new one with that .iso until the time your program has achieved compatibility with Windows 7. (Note most things designed for Vista will work on 7). Alternatively you may input a key at this time (its up to you whether it is worthwhile purchasing Vista to get the key).
What is the program?